The present invention relates to a new and improved method and apparatus for tapping a fastener having thread convolutions with different pitches.
An internally threaded fastener having coincident coarse and fine pitch thread convolutions with intermingled turns was invented by Harold James Mortus. This fastener is the subject matter of U.S. patent application Ser. No. 292,238 filed Aug. 12, 1981 and entitled "Dual Thread Fastener and Method of Making the Same".
The tapping of the coarse and fine thread convolutions of the fastener disclosed in the Mortus application can be performed with a complicated and expensive special tap. This tap must be designed to simultaneously tap coincident thread convolutions of different pitches. Although a special tap is relatively expensive, the use of a single special tap facilitates controlling the starting points for the coarse and fine thread convolutions.
Rather than using a relatively expensive special tap, it is contemplated that the dual thread fastener disclosed in the above-mentioned Mortus application could be formed by sequentially tapping the fastener with a pair of taps having different pitches. For example, a first tap having a relatively fine pitch could be used to tap a fine thread convolution in the fastener and then a second tap having a relatively coarse pitch could be used to tap a relatively coarse thread in the fastener. Although the sequential use of a pair of taps avoids incurring the cost of making a special tap, when a pair of taps are used difficulty may be encountered in having a single starting point for both the coarse and fine thread convolutions. The use of a single starting point for the coarse and fine thread convolutions is advantageous in that it tends to minimize a tendency for false starts when either a coarse or fine external thread convolution is turned into the internally thread fastener. Also, the use of a single start does not significantly reduce the strength of the tapped hole and prevents cross threading.